1. Field of the Invention
The present invention relates generally to a modem, and more particularly, to a modem that enables a host computer or data terminal equipment to communicate via a transmission line.
2. Description of the Prior Art
Host computers and data terminal equipment (hereinafter collectively referred to as the data terminal equipment) are interfaced to transmission lines oftentimes by means of modems. The modem modulates transmit data received from the data terminal equipment and sends out the modulated data to a transmission line. The modem also functions to demodulate signals transmitted from the transmission line, to recover the original data, and to transfer the recovered data to the data terminal equipment, thus enabling communication between data terminal equipment at different locations.
The modem is also capable of accepting input signals from the data terminal equipment as control commands in accordance with which the modem performs control processing such as setting communication protocols. However, since transmit data and control commands are both input to the modem using the same input line from the data terminal equipment, discrimination between the transmit data and control commands is not possible by the input signals only. Therefore, the conventional modem is provided with two modes, one being a control command accept mode for accepting an input signal as a control command and the other being a data transmit mode for accepting an input signal as transmit data, and the modem switches between these two modes according to the situation.
When the modem is in the control command accept mode in which any input signal from the data terminal equipment is accepted as a control command, switchover to the data transmit mode can be easily accomplished by sending a control command for mode switching. However, when the modem is in the data transmit mode, since all input signals from the data terminal equipment are accepted as transmit data, accomplishing the switchover to the control command accept mode is more difficult. For example, even when a specific identification code string is assigned as a control command for mode switching, there always remains the possibility that the transmit data, which can include any data, may contain a data pattern that agrees, even if merely by chance, with the identification code string. Therefore, it has not always been possible in the past to accomplish a reliably distinction between a control command and transmit data.
In conventional modems, one of the following two methods has been used to achieve the switchover from the data transmit mode to the control command accept mode:
(1) A specific identification code string (e.g., "+++") is assigned as a control command, and only when this identification code string is received from the data terminal equipment with a prescribed time (guard time) inserted before and after the code string the modem is switched over to the control command accept mode: or PA1 (2) A selector switch ( DATA pushbutton switch) is provided on the modem, and when this switch is pressed, the modem is switched over to the control command accept mode. When this switch is pressed again, the modem returns to the data transmit mode.
According to the above method (1) using a specific identification code string, the mode switchover is effected only when the reception of the identification code string is preceded and followed by an interruption of transmission for a prescribed time. This eliminates the possibility of mistakenly effecting the mode switchover even if the same data pattern as the identification code string happens to exist in the data being transmitted. On the other hand, according to the method (2) using a selector switch, since the modem is directly operated by a mechanical means, the mode switchover can be effected in a reliable manner.
However, the above prior art method (1) using a specific identification code string has the problem that it requires the provision not only of a code string detection function for detecting the identification code string contained in transmit data but also of a timer function for measuring the prescribed time inserted before and after the identification code string, which leads to an increase in the manufacturing cost of the modem. Another problem with this method is that, if the mode switchover is to be effected automatically by sending the identification code string by software, etc., the data terminal equipment also is required to have the function of measuring the time, which only complicates the software, etc., although such a problem does not occur when the operator manually operates the data terminal equipment.
On the other hand, the prior art method (2) using a selector switch has the problem that it requires the operator intervention to operate the selector switch. If the operation is to be performed automatically, special hardware has to be provided for both the data terminal equipment and the modem. Therefore, with such method, it is difficult to automate the operation.
In view of the above situation, there is a strong need in the art for a modem that can effect the switchover from data transmit mode to control command accept mode in a simple manner by just sending a specific identification signal from the data terminal equipment.
Accordingly, the present invention overcomes the aforementioned and other shortcomings of the above known and similar devices and methods for providing communication via a modem. The present invention is summarized and described in detail below.